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TORONTO – Men who have a vasectomy may be increasing their risk of prostate cancer, and more specifically, developing an advanced, lethal form of the disease, a new study is warning.

But the Harvard School of Public Health scientists are adding: the heightened risk is small. And experts in the field who are not involved in the large-scale study say more research needs to be done to prop up the findings.

“I would be cautious about applying these findings to clinical practice right now. This is not like cigarette smoking causing a large number of people to develop lung cancer. This is a small increase in the risk of prostate cancer,” Dr. Louis Kavoussi, chairman of urology at North Shore-LLJ Health System, told CBS News.

“Further studies really need to be mandated in a better controlled fashion,” he said.

Vasectomies are pretty simple, safe operations that block or cut the tubes through which sperm passes into the semen in men. It’s used as a permanent contraception, and according to the Cleveland Clinic, about five per cent of men at reproductive age have had the procedure done.

The Harvard study is based on the health data of more than 49,400 U.S. men who were followed for 24 years between 1986 and 2010. During that time, more than 6,000 cases of prostate cancer were diagnosed, including 811 deaths linked to the disease.

The researchers say the study is the largest study to date to look at the link between vasectomy and prostate cancer.

Twenty-five per cent of the men reported having a vasectomy during the quarter century timeframe. That group had a 10 per cent increased risk of developing prostate cancer compared to their peers, the study found.

The vasectomies weren’t linked to low-grade cancer, either. Instead, the procedure was tied to a 20 per cent higher risk of advanced prostate cancer and a 19 per cent increased risk in fatal prostate cancer. Even men who had received a regular PSA screening – a prostate specific test – were at a 56 per cent increased risk of coming across the cancer if they had a vasectomy. The effect appeared most in men who had a vasectomy earlier on in life – before the age of 38.

It’s unclear why this link exists, but experts say it might have to do with protein changes in semen.

But keep in mind, the risk in general was low, the researchers say: about 16 in 1,000 men developed aggressive prostate cancer during the 24 years. That’s less than two per cent.